Twenty years to the day since the Berlin Wall was first breached, the city will today play host to a celebration of the momentous changes brought about by the fall of a monument to a repressive regime.

Angela Merkel will be joined by a host of foreign dignitaries, including Gordon Brown and Hillary Clinton, in walking through the Brandenburg Gate; the pianist and conductor Daniel Barenboim will lead an orchestral performance; 1,000 giant dominos running along the path of the wall will be toppled in a rapid re-enactment of the end of Communism in Europe, a process that in reality took months.

Earlier in the day, Ms Merkel will take stock at a less glitzy location. The Bornholmer bridge is off the tourist track, a big, ugly, grey-painted web of steel girders, still flanked by the last remaining remnants of the Wall. Yet at around 3 this afternoon, Chancellor Merkel will be joined by Mikhail Gorbachev and Lech Walesa – two men who arguably did more than anyone else to bring an end the Cold War – for a brief pilgrimage to the site. Their gesture will be purely symbolic, but the anniversary would be unthinkable without it.

The Berlin Wall was first breached at the Communist-controlled east-west crossing on the Bornholmer bridge. It was the weak point in the 96-mile-long barrier that finally burst open. I was lucky enough to have witnessed it.

I had flown into Berlin for the BBC from London six days beforehand to find the city in a state of feverish apprehension. Everyone felt that something was going to happen, but the question was what? Nobody dreamt that the Berlin Wall would suddenly open, let alone that Germany would be reunited within a year. With hindsight, the assumption seems almost idiotic, but the widely held expectation was that East Germany would attempt to reform itself from within.

But then came the now-famous press conference given by Günter Schabowski, the East German Politburo member and ex-editor of the Communist Party newspaper Neues Deutschland. Mr Schabowski, yellow and utterly exhausted, sat on a podium before the world's media and read out utterly confusing details of the regime's draft travel law – including the announcement that the right to travel to the West came into force immediately.

Within about an hour the first West German reports started coming in about East Germans gathering at crossing points. We leapt into a car and raced to the centre of Berlin. The watchtowers and guard houses were manned with their customary Kalashnikov-toting sentries. There was nothing to report.

Following the Wall northwards, we wound through a maze of dimly lit streets until we began to see single people walking towards us. Single people turned into groups. We thought: "They are just inquisitive West Berliners taking a look." But then the groups turned into a small crowd and we had to park the car and start walking.

What confronted us at the Bornholmer bridge was beyond belief: a vast tide of people, some in tears, many just looking stunned, flooding across. Behind them were the huge glaring arc lights, guard huts, raised barriers and a line of puffing Trabant and Wartburg cars. Two East German border guards stood there looking bewildered. One, I remember, was crying. With no orders from above, the guards had simply buckled under the pressure of a 20,000 strong crowd of East Berliners chanting "Open the gate!" and raised the barriers.